1. Field of the Invention
The present invention relates to a dynamic pressure type fluid bearing device which is used in business equipment, video equipment, measurement equipment, and the like.
2. Description of the Prior Art
Equipment which uses a bearing device similar to this invention, for example, a scanner unit for a laser printer shown in FIG. 5, is known. In this scanner unit, one end of a shaft member 2 is secured to a base 1a of a housing. A sleeve 3 fitted about the shaft member 2 is supported radially through a dynamic pressure type radial fluid bearing formed by a bore surface of the sleeve 3 and an outer surface of the shaft member 2 which has a dynamic pressure generating groove formed in the outer surface thereof. A thrust receiver 5 having an exhaust bore 5a is attached to one end (the upper end in FIG. 5) of the sleeve 3. The sleeve 3 is supported axially by a dynamic pressure type fluid bearing constituted by a lower surface of the thrust receiver 5 and an end face of the shaft member 2 opposed to the lower surface.
The sleeve 3 has a flange 3a. A polygonal mirror 6 is fitted on an outer peripheral surface of the sleeve 3 above the flange 3a so that the polygonal mirror 6 is sandwiched between a fixing ring 7a and the flange 3a, respectively, at an upper surface and a lower surface of the polygonal mirror 6.
A window 8 of a transport material is provided at a position on a side wall 1b of the housing 1 opposing the polygonal mirror 6 in a horizontal direction.
Furthermore, the sleeve 3 has a step portion at a position below the flange 3a. An upper surface of a rotor magnet 9a which is fitted about the outer peripheral surface of the sleeve 3 is held by the step portion. A lower surface of the rotor magnet 9a abuts against a fixing ring 7b so that the rotor magnet 9a is sandwhiched between the step portion and the fixing ring 7b. A stator coil 9b radially opposing the rotor magnet 9a is attached to a side wall 1c of the housing 1. The rotor magnet 9a and the stator coil 9b constitute a peripheral-surface opposing type driving motor.
In the scanner unit described above, when the sleeve 3 is rotated by the operation of the driving motor while being supported by the shaft member 2, a laser beam incident onto the polygonal mirror 6 is reflected from the mirror 6 and passes through the window 8 so that an object (not shown), such as a photosensitive drum or the like, placed outside is illuminated.
In the bearing device described above, the shaft member 2 is stationary and the sleeve 3, accessory parts including the polygonal mirror 6 attached to the sleeve 3, and the rotor magnet 9a, etc., constitute a rotary member is inclusively.
Since the bearing device is used at high rotational speeds, it is an indispensable requirement to correct an unbalance in weight in a radial direction.
However, the correction of this radial weight unbalance is performed after the polygonal mirror 6, the rotor magnet 9a and the like are attached to the sleeve 3 by rotating the sleeve 3 about a center axis line while holding the sleeve 3 horizontally and holding the outer peripheral surfaces of opposite end portions of the sleeve 3. Accordingly, the outer peripheral surfaces of the sleeve 3 are references for the correction.
On the other hand, during use of the bearing device, the sleeve 3 rotates with respect to a reference position defined by the bore surface.
For this reason, both the bore surface and the outer peripheral surface of the sleeve 3 are required to be machined with high accuracy so that both axis lines of the bore surface and the outer peripheral surface coincide exactly with each other, that is, to insure coaxiality. As a result, a drawback is involved in that the machining cost is increased.
Furthermore, in a bearing device of this type, it is desired to reduce the weight of the rotary member as far as possible to reduce the thrust load, and at the same time, to reduce the starting time.
Moreover, a thrust bearing formed between the thrust receiver 5 and the shaft member 2, and the radial bearing formed between the sleeve 3 and the shaft member 2 are fluid bearings of the dynamic pressure type with a gas, such as air, used as a lubricating fluid. During rotation of the rotary member, although the sleeve 3 rotates in a non-contacting condition with respect to the shaft member 2, due to a pumping action of the dynamic pressure generating grooves 4 formed in an outer peripheral surface of the shaft 2, at the start and stop conditions of the rotary member, the sleeve 3 and the shaft 2 are in contact with each other. However, since the gas, such as air, used as the lubricating fluid is inferior in lubricating properties to other lubricants, at the start and stop conditions of the rotary members, considerable damage is caused on a radial bearing surface of the sleeve 3 which is the contact surface between the sleeve 3 and shaft 2, and on the thrust bearing surface of the thrust receiver 5.
In order to prevent such damage on the bearing surfaces, the sleeve 3 is formed of a metal material which is subjected to cutting work and applied in a film, such as by plating, so as to have an excellent sliding property. Furthermore, the thrust receiver 5 is formed by cutting a metal material and injection molding a synthetic resin having an excellent sliding property on a surface of the metal material integrally therewith.
However, applying such a surface treatment to the sleeve 3 and the thrust receiver 5 not only increases the manufacturing cost but a drawback is involved since the sleeve 3 and the thrust receiver 5 are separate members. Press fitting, shrinkage fitting, or the like is required in assembling which is troublesome and time consuming.
Furthermore, in a bearing device of this type, the polygonal mirror 6 is mounted by the fixing ring 7a used exclusively for this purpose. As a result, there is a drawback in that the number of parts increases.